In the related art, a steam turbine having a casing through which steam flows, and a shaft body rotatably provided inside this casing has been known as a type of a steam turbine. In this steam turbine, stator blades are fixed to an inner peripheral surface of the casing, rotor blades are fixed to an outer peripheral surface of the shaft body, and a plurality of stages of stator blades and rotor blades are alternately provided in axial direction.
This steam turbine is roughly classified into an impulse turbine and a reaction turbine depending on a difference in operation type. In the impulse turbine, the rotor blades are rotated only by an impulse force received from steam. In the impulse turbine, the stator blades have a nozzle shape, the steam passing through the stator blades are jetted to the rotor blades, and the rotor blades are rotated only by the impulse force received from the steam. Meanwhile, in the reaction turbine, the shape of the stator blades are the same as that of the rotor blades, and the rotor blades are rotated by an impulse force received from the steam passing through the stator blades, and by a reaction force against the expansion of the steam generated when passing by the rotor blades.
Incidentally, in such a steam turbine, gaps with a predetermined width are formed in a radial direction between tip portions of the rotor blades and the casing, and gaps with a predetermined width are also formed in the radial direction between tip portions of the stator blades and the shaft body. A portion of the steam that flows in an axis direction of the shaft body leaks to a downstream side through the gaps of the tip portions of the rotor blades or stator blades. Here, since the steam leaked to the downstream side from the gaps between the rotor blades and the casing does not give an impulse force or a reaction force to the rotor blades, the steam hardly contributes as a driving force that rotates the rotor blades irrespective of either the impulse turbine or the reaction turbine. Additionally, since the steam leaked to the downstream side from the gaps between the stator blades and the shaft body does not change in speed and does not expand even if the steam flows over the stator blades, the steam hardly contributes as a driving force that rotates the stator blades on the downstream side irrespective of either the impulse turbine or the reaction turbine. Accordingly, in order to improve the performance of the steam turbine, it is important to reduce the amount of leaking of the steam in the gaps of the tip portions of the rotor blades or the stator blades.
Thus, sealing fins are used as means for preventing the steam from leaking from the gaps of the tip portions of the rotor blades or the stator blades. When the sealing fins are used for, for example, the tip portions of the rotor blades, the sealing fins protrude from either the rotor blades or the casing, and are provided so as to form minute gaps between the sealing fins and the other of the rotor blades or the casing.
Incidentally, as the rotor blades, there are known rotor blades, each having a protrusion that protrudes toward the upstream side, and being provided on an upstream surface of a shroud that constitutes the tip portion of the rotor blade, that is, a surface against which a steam current collides (refer to Japanese Unexamined Patent Application Publication No. 2006-291967 and Japanese Unexamined Patent Application Publication No. 02-030903).
However, these Japanese Unexamined Patent Application Publication No. 2006-291967 and Japanese Unexamined Patent Application Publication No. 02-030903 do not describe the significance of providing this protrusion on the shroud.